TWO things are clear about human sexual orientation. First, it is biological; second, it is complex. Sexual behaviour, identity, attractions and fantasies don’t line up neatly. Consistently, biologists fail to recognise this.

In their 1948 book Sexual Behavior in the Human Male, Alfred Kinsey and his collaborators showed how male sexuality varies smoothly, from a majority identifying as completely heterosexual to a minority who identify as gay. Men “do not represent two discrete populations, heterosexual and homosexual”, wrote Kinsey. “The world is not to be divided into sheep and goats.” He concluded the same for women five …